In a mass spectrometer, an ion transport optical system, which is called an ion lens or ion guide, is used to converge ions sent from the previous stage, and in some cases accelerate them, in order to send them into a mass analyzer such as a quadrupole mass filter in the subsequent stage. One type of such ion transport optical systems conventionally used is a multipole rod type, such as a quadrupole or octapole system. FIG. 10(a) is a schematic perspective view of a general quadrupole rod type ion guide.
The ion guide 70 is composed of mutually-parallel four columnar (or tube-like) rod electrodes 71, 72, 73 and 74 which are arranged in such a manner as to surround an ion optical axis C. Generally, the same radio-frequency voltage is applied to two rod electrodes 71 and 73 facing across the ion optical axis C, and a radio-frequency voltage with the same amplitude and a reversed phase as the aforementioned radio-frequency voltage is applied to two rod electrodes 72 and 74 which are provided next to the rod electrodes 71 and 73 in the circumferential direction. The radio-frequency voltages applied as just described form a quadrupole radio-frequency electric field in the space surrounded by the four rod electrodes 71, 72, 73 and 74. In this radio-frequency electric field, ions can be oscillated and converged into the vicinity of the ion optical axis C, and then transported into the subsequent stage.
The applicant of the present patent application has proposed an ion transport optical system using virtual rod electrodes as illustrated in the perspective view of FIG. 10(b), as an ion transport optical system taking advantage of the favorable ion-converging capability of the multipole rod type ion guide, while also being capable of accelerating the ions (for example, refer to Patent Document 1 and other documents). In this configuration, the rod electrodes 71, 72, 73 and 74 illustrated in FIG. 10(a) are replaced by the virtual rod electrodes 75, 76, 77 and 78, each of which is composed of a plurality (four in the example of this figure) of electrode plain plates 79 arranged along the direction of the ion optical axis C.
In this virtual multipole rod type ion transport optical system, different voltages can be respectively applied to the four electrode plain plates 79 constituting one virtual rod electrode 75, 76, 77 or 78. Therefore, a direct current voltage which increases in a stepwise fashion toward the ion's traveling direction may be superimposed on the radio-frequency voltage in order to accelerate or, inversely, decelerate ions while they are passing through the space surrounded by the virtual rod electrodes 75, 76, 77 and 78. In addition, as illustrated in FIG. 10(c), the electrode plain plates 79 composing the virtual rod electrodes may be arranged so that they approach the ion optical axis C in a stepwise fashion along the ion's traveling direction in order to gradually narrow down the ion stream while converging it.
Although an ion transport optical system using virtual rod electrodes has excellent properties as just described, at the same time it also has disadvantages: since a rod which can be originally composed of one rod electrode is segmented into a plurality of electrode plain plates, the number of components is inevitably increased, which unavoidably leads to a cumbersome assembly and regulation in manufacturing and use. To solve these problems, the applicant of the present patent application has proposed a concrete configuration of a multipole rod type ion guide using a virtual rod electrode in Patent Document 2.
In the configuration disclosed in this document, one electrode plain plate has a slightly long shape with an arc-shaped end which faces the ion optical axis when the plate is completed. Four electrode plain plates which are arranged in a plane orthogonal to the ion optical axis in such a manner as to surround the ion optical axis form a single group, and four groups of electrode plain plates, i.e. sixteen electrode plain plates, which arranged in the ion optical axis direction to form the four virtual rod electrodes are screwed onto an insulating holder made of a synthetic resin. The electrode plain plates, to which the same voltage is applied, arranged to face across the ion optical axis in a plane orthogonal to the ion optical axis are connected by a short-circuiting line. Unitizing the main components including the electrode plain plates in this manner facilitates the handling of the components and reduces the time and labor for their assembly and regulation.
The number of electrode plain plates required in this configuration equals the total number of virtual rod electrodes multiplied by the total number of electrode plain plates constituting a virtual rod electrode. The number of components may not be serious for a quadrupole. However, with an increase of the number of poles, as in the case of a hexapole and octapole, the number of components increases, which leads to cumbersome assembly and regulation. This problem also arises in the case where the number of electrode plain plates constituting a virtual rod electrode is increased to ten, twenty or more for example. Given this factor, it is desired to further decrease the number of components and simplify the assembly and regulation. Furthermore, in the case where the individual electrode plain plates are individually screwed onto a holder, a subtle error in their relative position may occur, which might negatively affect the ions' convergency, i.e. ions' passage efficiency.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2000-149865    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2001-351563